Field of the Invention
The invention relates to an apparatus for the treatment of a flat steel product, taking place in throughput, having an indirectly heated annealing furnace chamber, having a conveyor device for continuously conveying the flat steel product over a conveyor path leading from an entry of the annealing furnace chamber to an exit of the annealing furnace chamber, and having nozzle arrangements for feeding atmosphere gas, which is reactive in relation to the flat steel product, into the annealing furnace chamber.
The invention also relates to a method for treating a flat steel product, in which the flat steel product is conveyed in continuous throughput through an indirectly heated annealing furnace chamber from its entry to its exit, an atmosphere which is reactive in relation to the flat steel product, and which is introduced into the annealing furnace chamber through nozzle arrangements, being maintained in the annealing furnace chamber.
Description of Related Art
When “flat steel products” are referred to here, this means rolling products consisting of steel, which are for example in the form of steel strip, sheet steel or cuttings obtained therefrom.
Inter alia, it is known from DE 25 22 485 A1 that the surface reactivity of steel strips can be conditioned in a controlled way by oxidation. Thus, even those flat steel products which cannot be coated with the necessary reproducibility and absence from defects in the untreated state, owing to the composition of their steel, can have a protective metal layer applied to them by hot-dip coating after controlled surface oxidation.
Products which may be coated in this way with such a layer protecting against corrosion include, for example, strips or sheets which consist of so-called “Advanced High Strength Steels” (AHSS). Besides iron and unavoidable impurities, such steels typically contain (in wt. %) C: 0.01-0.22%, Mn: 0.5-3.0%, Si: 0.2-3.0%, Al: 0.005-2.0%, Cr: up to 1.0%, Mo: up to 1.0%, Ti: up to 0.2%, V: up to 0.4%, Nb: up to 0.2%, Ni: up to 1.0%.
Owing to their industrial importance, many attempts have been made to carry out the pretreatment steps necessary for coating with the protective metal coat economically with the respectively available means.
A particular challenge in terms of plant technology in this case involves the preoxidation of flat steel products in indirectly heated continuous furnaces, so-called “Radiant Tube Furnaces”, abbreviated to “RTF”. In the case of furnaces of the RTF type, in contrast to furnaces in which an open flame is applied directly against the flat steel product to be treated and the oxidation potential in the atmosphere surrounding the strip in the furnace is influenced by modifying the gas/air mixture which is combusted, gas-heated burners are not used. Instead, the heating of the steel strip is carried out by means of heat radiators which are arranged along the conveyor path of the flat steel product through the annealing furnace chamber of the respective furnace.
In order to permit the desired oxidation of the surface of the flat steel product respectively to be treated in practice in an indirectly heated continuous furnace as well, it has been proposed in DE 10 2004 059 566 B3 to carry out the annealing in an RTF furnace in three steps. The first annealing step is in this case configured in such a way that diffusion of essential alloy constituents to the surface of the strip is substantially avoided. In the next step, an effective iron oxide layer, which prevents further alloy constituents from reaching the surface at the final elevated annealing temperature, is then formed in a controlled way. Thus, during the subsequent annealing treatment in a reducing atmosphere, a pure iron layer is formed which is very highly suitable for a surface-wide and firmly adhering coating of zinc and/or aluminium.
The method described above requires that the preoxidation takes place within an enclosed reaction chamber, into which for example O2 is fed as an oxidant. Generally, in the case of furnaces of the RTF type, the problem then arises of separating the annealing furnace chamber, in which the oxidation is intended to take place, in the region of its entry and exit respectively from the surroundings and a further annealing chamber, subsequently passed through, in which there is a different atmosphere. The challenge in this case consists in delimiting the mutually adjacent chambers of the annealing furnace from one another in such a way that the different atmospheres existing in the chambers are not contaminated by the other respective atmosphere to an extent exceeding a tolerance volume. If a reduction treatment is intended to be carried out in the chamber following the annealing furnace chamber in which the oxidation is carried out, then it is necessary to prevent both escape of the oxidant fed into the oxidation chamber into the reduction chamber and entry of the reducing atmosphere of the reduction chamber into the oxidation chamber. Otherwise, by means of undesired side reactions, the treatment result and consequently the result of the coating carried out after the annealing treatment may be lastingly impaired or it may become more difficult to control the individual annealing steps. Both restrict the process stability and may entail extra consumption of process gases.
WO 2009/030823 A1 discloses a possibility of introducing a gaseous oxidant by means of slotted or perforated steel tubes in a continuous furnace according to the RTF design.
JP 2003-342645 A furthermore describes an example of the way in which an enclosed oxidation zone can be achieved in an annealing furnace configured in chamber design. Undesired contamination of the reduction atmosphere by O2 is in this case intended to be avoided by a mechanical seal in the form of squeeze rollers and by a negative pressure inside the oxidation chamber. This method has the disadvantage that hydrogen unavoidably required for the reduction treatment is drawn from the reduction zone into the oxidation region. As a result, water is formed in the oxidation zone. This reaction binds oxygen present in the oxidation zone, which is consequently no longer available for the actually intended oxidation of the flat steel product surface. Targeted control of the oxidation of the flat steel product surface can therefore be achieved only with difficulty in practical use. In particular, it proves difficult to keep the plant power constant in the event of load changes. Wetting defects or deficient adhesion by the hot-dip coating can result from this. Furthermore, in the case of conventional input by means of slotted or perforated steel tubes, the oxidant has only a weak impulse and can therefore be carried away by the gas flow in the furnace interior before it reaches the flat steel product surface.